1. Field of the Invention
The present invention relates to an encoder.
2. Description of the Related Art
An optical encoder often uses a technique for acquiring waveforms of an A phase and a B phase from a four-phase sinusoidal wave and detecting a position based on Lissajous' curves obtained by drawing these waveforms. In the Lissajous' curves obtained by this technique, a problem of causing a position error due to harmonics of the third order or more is known.
As a technique for reducing the position error, for example, a technique for suppressing harmonics of the third order or more by optically filtering a component of the harmonics has already been proposed (JP-A-2007-248302).
Also, as another technique, a technique (JP-A-2010-216961) for geometrically analyzing and removing a component of harmonics has been proposed. In this technique, the harmonics of the third order or more can efficiently be removed by performing computation by a computing circuit.
However, the inventor found that the techniques described above had the following problems. In the technique described in JP-A-2007-248302, an influence of high-order harmonics already included in a signal is reduced. As a result, the influence of harmonics of the third order or more remains to some extent.
Also, the technique described in JP-A-2010-216961 requires complicated computation, so that a sophisticated computing circuit is required. Further, a delay is caused by computation time, so that a situation in which variations in harmonics cannot be followed and an influence of the harmonics cannot be removed occurs.
As described above, the techniques described above cannot implement an encoder capable of efficiently removing an influence of harmonics of the third order or more while following variations in the harmonics of the third order or more.